Freeze-­Drying of Probiotics for the Incorporation in Functional Foods: Drying Process, Viability, and Powder Properties

Abstract

Probiotics are defined as live microorganisms administered in adequate amounts as part of foods or supplements to benefit host health (Arellano et al. 2021). Since probiotics improve the intestinal microbial balance, foods containing probiotics are classified as functional foods. The beneficial impact of probiotics on human health includes alleviating inflammatory bowel disease, immune disorders, type 2 diabetes, and atherosclerosis (Arellano et al. 2021). Even though the daily dosage intake or the minimal viable cell number of probiotics is not well defined, it is suggested around 108 g−1, or ml−1, to exert health benefits in the gastrointestinal system (Her et al. 2015). Fermented foods (i.e. fermented dairy products) are natural sources of probiot ics. On the other hand, freeze-dried and spray-dried probiotic powders in capsules or sachets are available in the market, which is expanding rapidly. Probiotics in the package should maintain viability during transportation and storage. Different drying techniques have been used to obtain probiotic powders. The freeze-drying (FD) method has been preferred over others, such as spray-drying (SD), for higher viability and extended shelf life (Meireles Mafaldo et al. 2022). However, FD takes a long time and is more costly than spray drying. The drying conditions need to be optimized, and appropriate protective/cryoprotectant materials should be used to maintain the cell viability as high as possible during harsh conditions in drying processing steps such as pumping (shear and oxygen stresses),drying (shrinkage and ice crystals), growth to high cell densities, and grinding (shear and heat stress) after drying (Béal and Fonseca 2015; Her et al. 2015). Microencapsulation might be employed to protect the sensitive probiotic strains using FD and wall materials such as proteins (i.e. whey protein), alginates, poly saccharides, resistant starch, and milk, during processing and gastrointestinal stress conditions (Arellano et al. 2021). Maillard reaction products can improve the solubility, emulsifying properties, and thermal stability of proteins; hence pro tein and polysaccharide conjugates are functional wall materials for encapsulat ing probiotics (Li et al. 2023). The rehydration of dried probiotics is a vital step in producing functional foods. The rehydration conditions significantly influence the viability of the probiotics in the food matrix. This chapter aims to provide a general overview of the FD (lyophilizing) probi otics process, focusing on the significance of protective material, process optimi zation, characterization, and factors affecting the powder properties and viability of freeze-dried probiotics. It also reviews recent works incorporating freeze-dried probiotic powders into functional food products.